The present invention relates to controlling a permanent magnet synchronous machine.
In order to control a permanent magnet synchronous machine, it is known that it is necessary at all times to know the position of its rotor. Conventionally, position sensors are used to measure the position and to calculate the speed of the machine. The main drawback of using such sensors (and the processor cards that accompany them) is a reduction in the reliability of the system, where reliability is paramount in the field of aviation. Other drawbacks of that solution are the increases in the weight, in the volume, and in the total cost of the system.
A large amount of research work has therefore been performed in order to do without such a position sensor, and thus to estimate mechanical variables solely on the basis of measuring stator currents.
Several methods have already been proposed and validated for controlling a synchronous machine at medium and high speeds without using a position sensor. Those methods are based on estimating the no-load electromagnetic force (EMF) vector on the basis of imposed voltages, of measuring currents, and of equations describing the machine. Since EMF is directly proportional to speed, speed can also be estimated, as can position, which is then obtained merely by integrating the speed. Nevertheless, since the EMF is zero when the machine is stopped and since it is buried in measurement noise at low speeds, it is no longer observable in such operating ranges. Methods based on estimating EMF are therefore not suitable for applications in which position control is required.
In order to estimate position at low speeds and when stopped, the only remaining solution is to use variations in the values of stator inductances as a function of the position of the rotor. Several methods have already been proposed that make use of the variations in inductances:                In a first type of method, the principle is to switch off control even ten or twenty periods of pulse width modulation (PWM) and to inject a high frequency signal (at a frequency higher than the passband of the current regulators). The ratio of the injected voltage over the measured variation of current makes it possible to estimate inductance, and since inductance depends on position, it is also possible to estimate position. An example is described in the document by J. Kiel, A. Bünte, S. Beineke, “Sensorless torque control of permanent magnet synchronous machines over the whole operation range”, EPEPEMC, TP-053, Dubrovnik & Cavat, September 2002.        In a second type of method, the error in the position estimate is itself estimated in an initial stage. This error is regulated towards zero with the help of a corrector. The output from the corrector provides an estimate of speed, and by integration it is possible to obtain the estimated position of the rotor. In order to calculate the error of the estimate, a measure of the current immediately after injecting the high frequency (HF) signal is compared with the current that would theoretically be obtained if no HF signal had been injected.        
The methods of the first above-mentioned type have the following drawbacks:                The estimated position is calculated directly, so it therefore suffers from discontinuities each time it is calculated. Since the voltage references are calculated on the basis of the position of the rotor, the references will also be subjected to discontinuities, which gives rise to jolts of torque that can be harmful.        The need to stop control in order to make an estimate. Every ten or twenty periods of the PWM (depending on desired accuracy), one such period is devoted solely to injecting a high frequency signal for estimation purposes.        Under such circumstances, it is necessary to oversample the stator current when making the estimate.        
The methods of the second above-mentioned type suffer from the following drawbacks:                In those methods, a measured current is compared with a current that ought to be obtained theoretically. In order to be able to do that, it is necessary to have an accurate model of the motor if the methods are to converge properly. They therefore become dependent on uncertainties concerning the parameters of the machine, and also on variations in those parameters.        Furthermore, those methods apply only to smooth rotor machines.        
There therefore exists a need to improve the control of a synchronous machine at low and zero speeds.